Test Quality Skill (JUnit 5 + AssertJ)

Write high-quality, maintainable tests for Java projects using modern best practices.

When to Use

• Writing new test classes
• Reviewing/improving existing tests
• User asks to "add tests" / "improve test coverage"
• Code review mentions missing tests

Framework Preferences

JUnit 5 (Jupiter)

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Nested;
import static org.assertj.core.api.Assertions.*;

AssertJ over standard assertions

✅ Use AssertJ:

assertThat(plugin.getState())
    .as("Plugin should be started after initialization")
    .isEqualTo(PluginState.STARTED);

assertThat(plugins)
    .hasSize(3)
    .extracting(Plugin::getId)
    .containsExactly("plugin1", "plugin2", "plugin3");

❌ Avoid JUnit assertions:

assertEquals(PluginState.STARTED, plugin.getState()); // Less readable
assertTrue(plugins.size() == 3); // Less descriptive failures

Test Structure (AAA Pattern)

Always use Arrange-Act-Assert pattern:

@Test
@DisplayName("Should load plugin from valid directory")
void shouldLoadPluginFromValidDirectory() {
    // Arrange - Setup test data and dependencies
    Path pluginDir = Paths.get("test-plugins/valid-plugin");
    PluginLoader loader = new DefaultPluginLoader();
    
    // Act - Execute the behavior being tested
    Plugin plugin = loader.load(pluginDir);
    
    // Assert - Verify results
    assertThat(plugin)
        .isNotNull()
        .extracting(Plugin::getId, Plugin::getVersion)
        .containsExactly("test-plugin", "1.0.0");
}

Naming Conventions

Test class names

// Class under test: PluginManager
PluginManagerTest           // ✅ Simple, standard
PluginManagerShould         // ✅ BDD style (if team prefers)
TestPluginManager           // ❌ Avoid

Test method names

Option 1: should_expectedBehavior_when_condition (descriptive)

@Test
void should_throwException_when_pluginDirectoryNotFound() { }

@Test  
void should_returnEmptyList_when_noPluginsAvailable() { }

@Test
void should_loadPluginsInDependencyOrder_when_multipleDependencies() { }

Option 2: Natural language with @DisplayName (cleaner code)

@Test
@DisplayName("Should load all plugins from directory")
void loadAllPlugins() { }

@Test
@DisplayName("Should throw exception when plugin descriptor is invalid")
void invalidPluginDescriptor() { }

AssertJ Power Features

Collection assertions

// Basic collection checks
assertThat(plugins)
    .isNotEmpty()
    .hasSize(2)
    .doesNotContainNull();

// Advanced filtering and extraction
assertThat(plugins)
    .filteredOn(p -> p.getState() == PluginState.STARTED)
    .extracting(Plugin::getId)
    .containsExactlyInAnyOrder("plugin-a", "plugin-b");

// All elements match condition
assertThat(plugins)
    .allMatch(p -> p.getVersion() != null, "All plugins have version");

Exception assertions

// Basic exception check
assertThatThrownBy(() -> loader.load(invalidPath))
    .isInstanceOf(PluginException.class)
    .hasMessageContaining("Invalid plugin descriptor");

// Detailed exception verification
assertThatThrownBy(() -> manager.startPlugin("missing-plugin"))
    .isInstanceOf(PluginException.class)
    .hasMessageContaining("Plugin not found")
    .hasCauseInstanceOf(IllegalArgumentException.class)
    .hasNoCause(); // or verify cause chain

// With assertThatExceptionOfType (more readable)
assertThatExceptionOfType(PluginException.class)
    .isThrownBy(() -> loader.load(invalidPath))
    .withMessageContaining("Invalid")
    .withMessageMatching("Invalid .* descriptor");

Object assertions

// Extract and verify multiple properties
assertThat(plugin)
    .isNotNull()
    .extracting("id", "version", "state")
    .containsExactly("my-plugin", "1.0", PluginState.STARTED);

// Using method references (type-safe)
assertThat(plugin)
    .extracting(Plugin::getId, Plugin::getVersion, Plugin::getState)
    .containsExactly("my-plugin", "1.0", PluginState.STARTED);

// Field by field comparison
assertThat(actualPlugin)
    .usingRecursiveComparison()
    .isEqualTo(expectedPlugin);

Soft assertions (multiple checks)

@Test
void shouldHaveValidPluginDescriptor() {
    SoftAssertions softly = new SoftAssertions();
    
    softly.assertThat(descriptor.getId())
        .as("Plugin ID")
        .isNotBlank()
        .matches("[a-z0-9-]+");
    
    softly.assertThat(descriptor.getVersion())
        .as("Plugin version")
        .matches("\\d+\\.\\d+\\.\\d+");
    
    softly.assertThat(descriptor.getDependencies())
        .as("Dependencies")
        .isNotNull()
        .doesNotContainNull();
    
    softly.assertAll(); // All assertions evaluated, even if some fail
}

String assertions

assertThat(errorMessage)
    .startsWith("Error:")
    .contains("plugin", "failed")
    .doesNotContain("success")
    .matches("Error: .* failed")
    .hasLineCount(3);

Test Organization

Nested tests for clarity

@DisplayName("PluginManager")
class PluginManagerTest {
    
    private PluginManager manager;
    
    @BeforeEach
    void setUp() {
        manager = new DefaultPluginManager();
    }
    
    @Nested
    @DisplayName("when starting plugins")
    class WhenStartingPlugins {
        
        @Test
        @DisplayName("should start all plugins in dependency order")
        void shouldStartInDependencyOrder() {
            // Test implementation
        }
        
        @Test
        @DisplayName("should skip disabled plugins")
        void shouldSkipDisabledPlugins() {
            // Test implementation
        }
        
        @Test
        @DisplayName("should fail if circular dependency detected")
        void shouldFailOnCircularDependency() {
            // Test implementation
        }
    }
    
    @Nested
    @DisplayName("when stopping plugins")  
    class WhenStoppingPlugins {
        
        @Test
        @DisplayName("should stop plugins in reverse dependency order")
        void shouldStopInReverseOrder() {
            // Test implementation
        }
    }
}

Parameterized tests

@ParameterizedTest
@ValueSource(strings = {"1.0.0", "2.1.3", "10.0.0-SNAPSHOT"})
@DisplayName("Should accept valid semantic versions")
void shouldAcceptValidVersions(String version) {
    assertThat(VersionParser.parse(version))
        .isNotNull()
        .hasFieldOrPropertyWithValue("valid", true);
}

@ParameterizedTest
@CsvSource({
    "plugin-a, 1.0, STARTED",
    "plugin-b, 2.0, STOPPED",
    "plugin-c, 1.5, DISABLED"
})
@DisplayName("Should load plugin with expected state")
void shouldLoadPluginWithState(String id, String version, PluginState expectedState) {
    Plugin plugin = createPlugin(id, version);
    
    assertThat(plugin.getState()).isEqualTo(expectedState);
}

@ParameterizedTest
@MethodSource("invalidPluginDescriptors")
@DisplayName("Should reject invalid plugin descriptors")
void shouldRejectInvalidDescriptors(PluginDescriptor descriptor, String expectedError) {
    assertThatThrownBy(() -> validator.validate(descriptor))
        .hasMessageContaining(expectedError);
}

static Stream<Arguments> invalidPluginDescriptors() {
    return Stream.of(
        Arguments.of(descriptorWithoutId(), "Missing plugin ID"),
        Arguments.of(descriptorWithInvalidVersion(), "Invalid version format"),
        Arguments.of(descriptorWithEmptyId(), "Plugin ID cannot be empty")
    );
}

Common Patterns

Testing with mocks (Mockito)

@ExtendWith(MockitoExtension.class)
class PluginManagerTest {
    
    @Mock
    private PluginRepository repository;
    
    @Mock
    private PluginValidator validator;
    
    @InjectMocks
    private DefaultPluginManager manager;
    
    @Test
    @DisplayName("Should load plugins from repository")
    v